The present invention generally realtes to electric motor control techniques and more particularly relates to a method and circuit arrangement for generating a pulse-width modulated actuating signal for a d.c. actuator.
Electrical direct current actuators, which have a distinct low-pass behavior with respect to the electrical or electromagnetic subsystem, are frequently used in electronic control and regulation devices. Typical examples of such actuators are d.c. motors or electromagnets, for example of the kind applied for actuating an electric parking brake or an electrohydraulic pressure generator or for the external control of an electromechanically actuated brake-force booster.
In order to minimize the power losses when controlling such electric d.c. actuators, the associated power amplifiers usually are controlled by a pulse-width modulated actuating signal in the kHz frequency range. The associated pulse-width modulator consists of a saw-tooth voltage generator which generates a saw-tooth shaped signal with the specified, constant kHz frequency and a maximum amplitude that is equal to the value of a reference voltage and a comparator that compares the saw-tooth signal with the value of a continuous actuating signal derived from the control circuit and fluctuating between a maximum and minimum value. When a d.c. motor is the actuator, the actuating signal, for example, is the armature voltage to set the desired torque of the motor. The reference voltage corresponds to the nominal value of the supply voltage; when this is used in a motor vehicle, the supply voltage generally is equal to the battery voltage of the vehicle electric system.
Furthermore, the comparator of the pulse-width modulator is designed in such a way that it supplies a binary switching signal for at least one switch arranged in the d.c. circuit of the actuator when the saw-tooth signal is below the actuating signal and uses this to generate a pulse-width modulated actuating signal with a pulse width representing the value of the continuous actuating signal and a pulse amplitude corresponding to the supply voltage within a specified pulse duty factor. Due to the low-pass behavior for the electric or electromagnetic subsystem of the actuator, a medium continuous actuator current of a specified size for activating the actuator according to the specifications from the control system adjusts itself.
The principle of the mode of functioning of such a pulse-width modulator is shown in FIG. 4. Section A shows the course of the continuous actuating signal UA in relation to time. When a d.c. motor is the actuator this actuating signal UA, for example, is the armature voltage of the motor. The size of the actuating signal is determined in a superior control module, e.g. a microprocessor, in such a way that the d.c. actuator is activated with the necessary, desired actuating value.
Section B shows the form of the saw-tooth signal with respect to the actuating signal UA, with the maximum amplitude of the saw-tooth signal being equal to the reference voltage (URef) whichxe2x80x94when applied in a motor vehiclexe2x80x94usually corresponds to the battery voltage of the vehicle electric system UBat,N with the nominal value UBat,N=12 Volt.
The pulse-width modulator generates a binary switching signal for at least one switch arranged in the d.c. circuit of the d.c. actuator, depending on whether the saw-tooth signal lies below or above the actuating signal UA.
FIG. 5 shows two typical actuators and their d.c. circuits, which are operated by the supply voltage, in the example shown by the battery voltage of the motor vehicle. Part A shows a d.c. motor M for a bidirectional movement, for example for actuating an electric parking brake. Four switches S1 to S4 are shown, which symbolically represent the associated power transistors that are controlled by the mentioned switching signal PWM and a negated switching signal PWM.
Part B shows the control of an electromagnet, for example in the external actuation of an electromechanically actuated brake-force booster, by means of switch S that is switched by the mentioned switching signal.
The ON duration Tein and the OFF duration Taus of the switches is shown in Part C of FIG. 4. This diagram shows the pulse-width modulated actuating signal which leads to a medium actuator current (not shown) having the specified size in the d.c. circuits of the actuators due to the mentioned low-pass behavior.
For the derivation, in particular for calculating the continuous actuating signal, as well as for the configuration of the pulse-width modulator it is generally assumed that the supply voltage to be switched by the pulse-width modulator for activating the actuator, in the example the battery voltage of the vehicle electric system UBat, is always constant and corresponds to the nominal value of the battery voltage UBat,N=12 Volt. Thus, the same pulse duty factor p=Tein/T always is obtained for a defined actuating signal UA. In this connection, the time T stands for the period of the saw-tooth signal. Assuming that the supply voltage UBat that is to be switched corresponds to its nominal value UBat,N, the following correlation then applies for the actuating signal UA:
UA=Pxc2x7UBat=Pxc2x7UBat,Nxe2x80x83xe2x80x83(1)
For determining the pulse duty factor p, especially on a microcontroller, the nominal value of the supply voltage UBat,N that is to be switched is taken as the basis. If the actually switched supply voltage UBat does not correspond to its nominal value UBat,N, as was assumed for calculating the pulse duty factor p, then the pulse duty factor p and, hence, the pulse width will remain constant; however, the pulse amplitude will deviate from its nominal value, namely Ubat,N. Due to Gl. (1), therefore, a voltage value UA deviating from the specified value UA results in the actuator circuit on average, and the actuator is actually controlled with this.
Consequently, the actuating value of a superior control system is not realized exactly in this case, since the actuator is controlled with a value differing from the actuating signal. This leads to a deterioration of the control quality which becomes more and more serious as the actual supply voltage deviates increasingly from the nominal voltage. In individual cases, the control circuit may not be able to set a specified value or may tend to exhibit instabilities.
The object of the invention is to design the above-mentioned circuit arrangement for generating a specified pulse-width modulated actuating signal for a d.c. actuator in such a way that the pulse duty factor is adapted to the actual value of the supply voltage, wherein such circuit has a pulse-width modulator consisting of a saw-tooth voltage generator that generates a relatively high-frequency saw-tooth signal with a constant frequency and a maximum amplitude that is equal to the value of the supply voltage serving as the reference voltage as well as a comparator, to which the saw-tooth signal and a continuous actuating signal having a predefined size can be applied as input signals and which is designed in such a way that it supplies a first binary switching signal for at least one switch that is arranged in the d.c. circuit of the actuator connected to the supply direct voltage when the saw-tooth voltage is below the actuating signal and it supplies a second negated binary switching signal when the saw-tooth voltage is above the actuating signal and which uses this to generate a pulse-width modulated actuating signal with a pulse width representing the value of the continuous actuating signal and a pulse amplitude corresponding to the supply voltage within a specified pulse duty factor.
This object is achieved by a method according to the invetion in that the actual value of the supply voltage is determined and the pulse duty factor is adapted to the actual value of the supply voltage.
For this purpose, preferably a nominal value of the supply voltage is divided by the actual value of the supply voltage in order to form a scaling factor that is used to rescale the continuous actuating signal.
A suitable circuit arrangement for executing the method is characterized in that a first functional module is provided, which calculates the scaling factor. A second functional module, which is arranged downstream of the first functional module, multiplies the continuous actuating signal by the scaling factor and feeds the result to the pulse-width modulator.
Thus, in the method according to the present invention and the circuit arrangement for executing the method, the actuating signal derivedxe2x80x94and in particular calculatedxe2x80x94from the control circuit and in particular calculated, is not fed directly to the pulse-width modulator, but this actuating signal is multiplied by a scaling factor before it is. fed to the pulse-width modulator. This resealing of the actuating signal ensures that the pulse duty factor for the required actuating signal is set in such a way that the actuator voltage applied to the actuator by means of the pulse-width modulator always corresponds to the desired value irrespective of the supply voltage that is to be actually switched. This is true both when the actual supply voltage is equal to the nominal voltage and when the supply voltages vary more or less greatly from the nominal voltage.
According to a further embodiment of the invention, the functional modules of the circuit arrangement preferably are formed by program modules of a microprocessor which, as a rule, is contained in the superior control systems anyway.
In another advantageous further embodiment of the invention, the actuating signal is first fed to a limiter that limits the actuating signal in both directions before it is rescaled by multiplying it by the scaling factor.
The method according to the present invention and the circuit arrangement for executing the method preferably are used in electronic systems in motor vehicles. Thus, according to an embodiment of the invention, the actuator preferably is a d.c. motor which, for example, is used for actuating an electric parking brake or an electrohydraulic pressure generator.
According to another embodiment of the invention the actuator is an electromagnet used for the external actuation of an electromechanically actuated brake-force booster in a motor vehicle.
In all these applications in motor vehicles, the supply voltage preferably should be the battery voltage of the vehicle electric system.